A water-absorbing resin (SAP/Super Absorbent Polymer) is a water-swelling and water-insoluble polymer gelling agent and has been used for mainly disposable uses including absorbing articles such as paper diapers and sanitary napkins and the like, further, water retention agents for agriculture and horticulture, and water-stopping materials for industrial use and the like (Non-patent Literature Document 1). For these water-absorbing resins, a number of monomers and hydrophilic polymers have been suggested as raw materials. However, a polyacrylic acid salt type water-absorbing resin made mainly from an acrylic acid is mainstream thereof and most of the water-absorbing resins manufactured all over the world are crosslinked polymer of polyacrylic acid sodium salt. And polyacrylic acid (salt)-type water-absorbing resins are proposed by Patent Documents 1 to 8 and the like. Acrylic acid used for these polymerizations are, in general, obtained by vapor-phase oxidation of propylene from petroleum (fossil material).
Such a water-absorbing resin can be obtained by finely granulating a water-containing gel-like polymer, which is obtained by polymerizing an aqueous monomer solution, during or after the polymerization, and drying the obtained particulated water-containing gel-like polymer. After drying, if necessary, a pulverizing step and a classification step are carried out and arbitrarily, surface-crosslinking is carried out before drying or after drying. Also, one or two or more of steps such as a fine powder recovery step, an undried matter removal step after drying, a packaging step, and an addition step of adding other additives (fine particles, a deodorant, an antibacterial agent, and the like) and the like may also be carried out. A method to be employed as a common polymerization method is aqueous solution polymerization or reverse-phase suspension polymerization and the product form is generally a white powder of about 10 to 1000 μm. Such a process for producing a water-absorbing resin involving the many steps is exemplified in Non-patent Literature Document 1 and Patent Documents 9 to 21 and the like.
And now, most of commercialized water-absorbing resins have a polymer structure of polyacrylic acid sodium salt and are insoluble in a solvent, so that it is impossible to measure their molecular weights as polymers. Also, since having no melting point, glass transition point (Tg), and the like, the water-absorbing resins cannot be distinguished from one another as polyacrylic acid (salts).
Accordingly, in the case where produced water-absorbing resins are actually used for disposable diapers, sanitary napkins, greening humectants, humectants for agriculture and horticulture, digestion gel, and the like after distributed through various distribution channels and subsequently discarded, it is difficult to determine in which factories, which plants, and which production lines such water-absorbing resins and their used hydrogels are produced and even if a certain water-absorbing resin causes a trouble at the time of its use or after its use or even if it is tried to assess the effect on environments, degradability in the ground after landfill or the like, it is difficult to distinguish which water-absorbing resin manufacturer, which factory, which plant, or which production line produced the water-absorbing resin.
The following Patent Documents 18 to 21 disclose such troubles (shutdown) in a pulverizing step, a transporting step, and a storage step. Many methods for producing various kinds of water-absorbing resins as disclosed in the following Patent Documents 1 to 21 have been proposed so far, but they do not disclose a method for solving all the troubles which possibly occur in the process for producing a water-absorbing resin.
Consequently, when a trouble occurs, the production method is revised and in general, the process for producing a water-absorbing resin involves many steps such as a monomer preparation step, a polymerization step, a drying step, a pulverizing step, and a surface-crosslinking step and in some production methods, many different raw materials are used and the production proceeds following a complicated course through many steps, and therefore it is difficult to pursue a cause of a trouble. Also, there are troubles which are discovered after time passes, such as coloration with the lapse of time of a water-absorbing resin as a representative, so that it is difficult to investigate the cause from the production method.
Regarding water-absorbing resins commercialized and distributed after production, there is a method of taking a water-absorbing resin out of a disposable diaper and analyzing the physical properties of the resin as a method for identifying a manufacturer and a distributor. Water-absorbing resins are controlled in accordance with many physical properties as shown in Patent Documents 1 to 21 etc. In recent years, the physical properties required by users represented by disposable diaper manufacturers are more or less similar, and therefore, even if water-absorbing resins are taken out of disposable diapers etc., their physical properties are often similar and consequently, it is difficult to improve the precision of the identification of a manufacturer and a distributor based on the water-absorbing resins. Still more, even if the physical properties (e.g., water absorption rate and water absorption capacity under load) of water-absorbing resins are the same, the measured values are fluctuated by about—several after the decimal point to one point several after the decimal point [g/g] and even if the physical properties of commercialized products are measured, the identification accuracy is low. It is conceivable to add a tracer substance to a water-absorbing resin; however, addition of a tracer not only causes an adverse effect on the cost and safety but also sometimes deteriorates the performance and whiteness of the water-absorbing resin. Also, such a problem is found occurring not only in the case of a water-insoluble and water-swelling polyacrylic acid (salt) (also referred to as a polyacrylic acid (salt)-type water-absorbing resin) but also in the case of a water-soluble polyacrylic acid (salt) (also referred to as a water-soluble polymer).
Also, a water-absorbing resin has another problem that the ratio of acrylic acid is high, in other words, the cost of a water-absorbing resin significantly depends on the price of propylene (petroleum). Also, in addition to the above-mentioned problem, since the water-absorbing resin containing acrylic acid as a main component is mainly used for disposal use such as disposable diapers and the like, the water-absorbing resin has another issue of stable supply of a raw material in terms of the cost. To deal with that, there is a technique known for obtaining a water-absorbing resin, for example, a carboxymethyl cellulose crosslinked body and the like by crosslinking starch or cellulose, but the physical properties are inferior as compared with those of a polyacrylic acid (salt).